I'm not sure I understand your question.
The second-person pronoun ty in Czech is always singular, while the English "you" can be either singular or plural. In English, you say "you were" when speaking to one person and when speaking to more than one person. "You was" is not, as VladaFu says, grammatically incorrect; it is not accepted.
In Czech, you decide whether to use ty jsi (singlular) or vy jste (plural) based on the number of people you're talking to (and whether your relationship is more formal or informal, if you're talking to one person). In some English-to-Czech sentences in the course, we don't have that kind of "relationship" information, so both ty jsi and vy jste may be accepted in translations.
It is not incorrect. In most cases, “you were strange then” would be the natural order, but it could be used for emphasis, e.g.:
— You were then strange (as a consequence of something, but now you're not).
I agree that it is not very natural though. I might use it like that in literary writing but not in speech. Unless I'm bidding for time while I think of the adjective:
— You were then… strange.